1. Field of the Invention
The present invention relates to a nickel, chromium, iron, aluminum, niobium, titanium welding alloy, articles made therefrom for use in producing weldments, and weldments and methods for producing these weldments. The present invention relates to Ni—Cr alloys useful as weld overlays applied for the purpose of enhancing corrosion resistance and, more particularly, where corrosion resistance in his temperature sulfidizing-oxidizing environments is a life-limiting factor.
2. Description of Related Art
In various welding applications including boiler waterwall tubing and reheater and superheater tubing, weld overlays are required to provide long-term corrosion resistance including resistance to corrosion fatigue cracking. The types of resistance requirements include sulfidation, carburization and coal ash corrosion resistance over a range of temperatures of 700° F. through 1450° F., which includes service in ultra-supercritical environments.
Prior to the initiation of NOx (oxides of nitrogen) control, boiler waterwalls did not require weld overlay and performed well when low alloy steels containing small amounts of chromium and sometimes molybdenum were used. Likewise, high-carbon austenitic stainless steel superheater and reheater tubes often performed well before the advent of low NOx boilers.
When environmental concerns dictated the need to reduce NOx emissions, coal-burning power plants began to install low-NOx burners and rationed the overall amount of air used for combustion. This resulted in a reducing environment firing condition within these boilers, the formation of H2S instead of SO2, and greatly increased corrosion rates of the boiler tubes. Protective weld metal overlays were chosen to extend the lives of both waterwall tubes and superheater and reheater tubes. Generally, overlays deposited with nickel-chromium-molybdenum alloy welding products were used until corrosion-fatigue failures became evident.
The next generation of weld overlays to be used was the molybdenum-free, nickel-chromium alloys that contained between 30-44% chromium. Superheater and reheater tubes seem to be performing well with 40-44% chromium-balance nickel overlays even in slightly reducing, carburizing and sulfidizing environments created by “supertuning”. However, waterwall tubes exposed to sulfidation in lower partial pressures of oxygen required greater protection during the most heavily reducing burn times. The present invention improves upon the current 40-44% chromium-balance nickel materials via additions of aluminum in the range of 0.5% to 2.0% and niobium in the range of up to 2%, in the interest of providing additional enhancements to corrosion resistance while maintaining the same degree of fabricability and usability as currently available materials.
Given the combination of high chromium content with added aluminum, with a nickel base, the alloy material of the invention is expected to find application for environments requiring resistance to metal dusting corrosion as well. Applications associated with production of syngas, consisting primarily of hydrogen and carbon monoxide, will be of primary interest.
The present invention overcomes the limitations of the prior art by providing a nickel, chromium, iron, niobium, titanium, aluminum welding alloy and weldments made therefrom that provide the desired corrosion resistance in addition to resistance to hot cracking, as well as corrosion fatigue cracking. The present invention further provides a welding alloy of the nickel, chromium, iron, titanium, aluminum type that is particularly adapted for use in fabricating equipment used in low NOx, coal-fired power generation.
It is a specific object of the present invention to provide a nickel, chromium, iron, titanium, aluminum welding alloy and weldments made therefrom that provide desired resistance to corrosion and corrosion fatigue under conditions of low partial-pressures of oxygen.
A further object of the invention is to provide a welding alloy of the nickel, chromium, aluminum type that is particularly adapted to fabricating and overlaying equipment, such as tubes, used in low NOx coal-fired power boilers.